It is known to test a continuous wave microwave doppler radar using a tuning fork designed to resonate at a given frequency to simulate a given speed of a moving object. Tuning forks eventually lose calibration particularly if damaged and need to be re-calibrated or replaced. Furthermore, a tuning fork is not easily modulatable in being able rapidly to be turned on or off. It is also known that some radars have built in calibration devices which can produce an electronic signal of a predetermined frequency. Such a calibration instrument can be easily turned on or off but it does not test the entire doppler radar system including the radar antenna.